Polycythemia or erythrocytosis may be defined as an elevation of the hemoglobin or hematocrit. The primary objective in the laboratory evaluation of polycythemia is to determine whether polycythemia vera is present, and to distinguish it from relative polycythemia (an elevated hemoglobin secondary to plasma volume depletion, with normal red cell mass), and secondary erythrocytosis (erythropoietin-mediated increase in red cells). Traditionally, this work-up has included the laboratory measurement of red cell mass. In fact, demonstration of an increased red cell mass was included in the diagnostic criteria for polycythemia vera established by the Polycythemia Vera Study Group 25 years ago.

Measurement of red cell mass is expensive, time-consuming and cumbersome. Recently, it has become evident that this procedure may not always be necessary or reliable in the evaluation of polycythemia. Firstly, measurement of red cell mass is unnecessary if the hemoglobin is higher than 18.5 g/dL in men and 16.5 g/dL in women, since this degree of hemoglobin elevation is almost invariably associated with an increase in red cell mass. Secondly, patients with polycythemia vera who have early disease or concurrent iron deficiency may not fulfill the criterion of an increased red cell mass. Finally, relative polycythemia appears to be very rare, and some studies have questioned its existence as a distinct entity. Consequently, some authorities recommend that the presence of certain polycythemia vera-related clinical and laboratory features should dictate further laboratory evaluation of polycythemia, regardless of the red cell mass. These features include persistent leukocytosis or thrombocytosis, microcytosis, splenomegaly, pruritis after bathing, unusual thrombosis, and erythromelalgia.

An approach to the laboratory evaluation of polycythemia is shown in the accompanying algorithm, which includes the following tests. This algorithm allows evaluation of polycythemia without an absolute requirement for measurement of red cell mass. In selected cases however, where findings are equivocal, measurement of red cell mass may still be appropriate.

Serum erythropoietin (EPO) level

Polycythemia vera is a neoplastic clonal blood disorder with autonomous (EPO-independent) erythroid proliferation, resulting in a negative feedback down-regulation of EPO production, and low (or normal) serum EPO levels. In contrast, secondary erythrocytosis is EPO-driven and characterized by elevated (or normal) serum EPO levels. An EPO assay is indicated if the hemoglobin level is greater than 18.5 g/dL in men or 16.5 g/dL in women, or if there is a lesser degree of hemoglobin elevation associated with one of the polycythemia vera-related features listed above. Polycythemia vera is a diagnostic possibility only if the EPO level is low or normal. A low serum EPO level indicates that polycythemia vera is very likely (a repeat EPO assay may be warranted to confirm the low value), and a bone marrow examination is indicated. A bone marrow should also be considered if the EPO level is normal, and one of the polycythemia-related features is present. If the serum EPO level is normal and polycythemia-related features are absent, the possibility of secondary erythrocytosis should be investigated, although polycythemia vera cannot entirely be ruled out. If the EPO level is elevated, the likely diagnosis is secondary erythrocytosis.

Arterial oxygen saturation

In the investigation of secondary erythrocytosis a decrease in arterial oxygen saturation is diagnostic of hypoxic erythrocytosis, which may be secondary to chronic pulmonary disease or cyanotic congenital heart disease.

Red cell oxygen dissociation (P50)

This test is indicated if the arterial oxygen saturation is normal. If the P50 value is decreased, the diagnosis may be high oxygen affinity hemoglobin. Hemoglobin electrophoresis should be obtained, however a normal result does not rule out the diagnosis. If the P50 is normal, the diagnosis may be inappropriate EPO secretion, which may be secondary to renal disease or a neoplasm.

Polycythemia 2

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